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Details of Horizontal Engine.                   215

form is used instead (p. 750), combining both internal and ex-
ternal gauge. For interchangeable work high and low gauges are
required, varying in size by a very slight but known amount, and
the aim is to make the work lie somewhere between the two, so
that any pair of parts will then fit, and the ' play' between them
never be more or less than certain fixed values. (See^. 277 / also
Appendix L, $. 750.)

Details of Horizontal Engine.—Having fully described
machines, tools, and general operations, we shall now proceed
to apply the information obtained to enable us to take piece by
piece the various parts constituting a 20 Horse-power Non-
condensing Engine, with automatic expansion gear; and, having
received such parts in the rough condition from the Smith or
Moulder, to follow them through their various stages, until put
together by the Erector to form the complete work. That course
has been thought advisable in dealing with this, the most impor-
tant chapter in Part I, in order to avoid any risk of omitting a
good example; it being supposed that if a student could
thoroughly discuss the whole of this machine he might be con-
sidered reasonably capable of thinking out any new case that
might be placed before him. In order to avoid repetition we will
make a few premises.

The Marker-off either chalks or white-washes his work before
commencing, and obtains the height for his scriber point by
first marking the same on the block B, Fig. 192, and then setting
the point to this mark. He should know something of the allow-
ances made by Smith and Pattern-maker, which are usually J in.
all over machined surfaces, and in extreme cases ^ in. Bed
plates, for example, warp \ in. or even more, and special material
must be left on their seatings,

Machining is marked on drawings to indicate all tooled
surfaces; being shewn by red lines; but in our case a thick
dotted line will serve the same purpose, thus: «• ••« — «•«•.
Further, although such drawings are copiously and fully supplied
with dimensions, these will be omitted in our examples, the scale
being given instead. The sizes represented on the drawing are
known as * finished sizes/ and the allowance on machined parts is
left to the judgment of the Pattern-maker or Smith.

In drilling, there are at least three various sizes that a hole